In general relativity, an event horizon (EH) is a region in spacetime beyond which events cannot affect an outside observer. In layman's terms, it is defined as the shell of "points of no return", i. e. , the boundary at which the gravitational pull of a massive object becomes so great as to make escape impossible. An event horizon is most commonly associated with black holes. Light emitted from inside the event horizon can never reach the outside observer. Likewise, any object approaching the horizon from the observer's side appears to slow down and never quite pass through the horizon, with its image becoming more and more redshifted as time elapses. This means that the wavelength of the light emitted from the object is getting longer as the object moves away from the observer. The travelling object, however, experiences no strange effects and does, in fact, pass through the horizon in a finite amount of proper time.